The roles of retroviral and host genes in the congenital transmission of avian leukosis viruses (ALVs) will be studied to learn more about the interactions of retroviruses with their hosts. Recombinant DNA technology will be used to generate recombinants of RAV-O (a virus that does not undergo congenital transmission) and RAV-1 (a virus that does undergo congenital transmission) to rigorously define the region of the ALV genome that determines ability to undergo congenital transmission. DNAs from oviducts infected with viruses that do and do not undergo congenital transmission will be analyzed to determine whether the step in the retroviral life cycle that restricts congenital transmission occurs before integration of proviral DNA into oviduct DNA. The replication of viruses that do and do not undergo congenital transmission will be examined in various tissues to learn if the "oviduct restriction" occurs in other tissues. Recombinant DNA technology will be used to construct a vaccine virus. Specifically, sequences encoding the type-specific antigens of a subgroup A ALV will be substituted for the comparable sequences of RAV-O ub RAV-O. Such a virus should be immunizing for subgroup A ALVs while undergoing benign, self-limited infections. The K28 line of chickens will be maintained and further selected for endogenous viruses and susceptibility to endogenous virus infections.